1. Field of the Invention
The present invention relates generally to a novel process and apparatus for the strengthening of film and, more particularly, pertains to a process and apparatus for the multidirectional orientation of film to improve the overall stiffness and tear resistance therein.
The inventive arrangement effectively provides a novel process and apparatus for the continuous multidirectional orientation of film which improves the overall stiffness and tear resistance thereof. By laminating a multidirectional oriented film to a similarly produced film, a laminated film having vastly improved qualities is formed.
2. Discussion of the Prior Art
The present state of the art related to the strengthening of films or laminates has developed techniques for uniaxially or biaxially orienting films to improve overall strength, stiffness and tear resistance. It is well known in the prior art that by stretching a film in one direction, this uniaxially orients the molecules of the film in the direction of stretch. By doing so, improved tear resistance and stiffness properties result in the direction transverse to the axis of stretching. Similarly, biaxial orientation can be accomplished in numerous ways. Particularly, a film can be simultaneously stretched along two axes thereby providing improved stiffness and strength in two distinct directions. Another method of providing a biaxially oriented laminated sheet is by uniaxially stretching one sheet and laminating it to a oppositely oriented uniaxial sheet. This provides a composite biaxial oriented film. Still, another way to biaxially orient a sheet is to uniaxially orientate the film, slit the film along a bias, and then laminate the film in such a manner as to have the orientation of one ply be oppositely disposed to the orientation of the second ply.
As is also well known in the prior art, numerous problems are encountered in seeking to achieve a laminate with improved overall stiffness and tear resistance. Although a uniaxial ply improves tear resistance transverse to the direction of stretching, tearing is easily effectuated along the longitudinal axis of stretching. Biaxial orientation improves stiffness and tear resistance in two directions, but the laminate is still highly susceptible to tears which run longitudinally along the combination of the axes. Further, the biaxial orientation methods used are not easily adaptable to on-line usage consequently making the prior art biaxial methods highly time consuming and very expensive.
The present invention, however, effectively alleviates the aforementioned problems related to the orientation of the molecules within the film. None of the prior art arrangements directed to solving these problems do so as completely and effectively as the present invention. None of the prior art, of which Haley U.S. Pat. No. 4,101,625; Takahasi U.S. Pat. No. 3,746,608; Kim et al. U.S. Pat. No. 4,101,358; Rasmussen U.S. Pat. No. 4,039,364 and Schwarz U.S. Pat. No. 4,223,059 are typical, show or even suggest the process and apparatus as described herein.
As is illustrated in Haley U.S. Pat. No. 4,101,625 the invention therein discloses a method for making molecularly oriented plastic strapping having improved physical properties which include increased resistance to longitudinal splitting and increased longitudinal stiffness. Haley provides a band formed of essentially non-oriented thermoplastic material having a rectangular cross-section which is of a corrugated configuration and is expanded in the band width direction so as to orient polymer molecules in the transverse direction after which the band is elongated to orient the polymer molecules in the longitudinal direction. The band of thermoplastic polymeric material is formed with an essentially flat, rectangular cross-section and is then compressed between a pair of cooperating corrugated rollers and is then expanded laterally so that the transverse section of the band has a corrugated configuration and the transverse orientation of the band of polymer molecules are achieved simultaneously. The produced longitudinally corrugated band is then stretched longitudinally to provide longitudinal orientation of the molecules within the band. Unlike the present invention, this patent teaches the use of corrugating bands intermittently within the film to supply increased stiffness and tear resistance coupled with a simultaneous stretching procedure. The present invention has nothing to do with corrugating film, but rather solely deals with orienting the molecular structure within the film in a relatively random fashion thereby increasing tear resistance in all directions.
Takahashi U.S. Pat. No. 3,746,608 discloses an elongated strapping band having high tensile strength and improved resistance to longitudinal splitting wherein the band is formed of polypropylene which has been longitudinally oriented and laminated to outer layers wherein such layers have been biaxially oriented by applying a transverse force sufficient to permanently change the initial uniaxial orientation to biaxial orientation. Finally, mechanically made indentations are pressed into the film to further distort the biaxial orientation of the molecules. This reference is highly indicative of the prior art in that it shows the mere combination of biaxial and uniaxial orientations to provide an improved film. The present invention is a clear departure of the art because of the randomness of the orientation which is imparted to the molecules of the film without resort to indentations or the like.
In Kim et al. U.S. Pat. No. 4,101,358 there is disclosed a method of making non-woven fabric by laminating sheets of parallel continuous main ribs interconnected by webs of reduced thickness extending therebetween and oriented at different angles thereto. This produces a structure reinforced by the differently-oriented ribs of the laminated layers. This mechanical deformation of the film to produce increased strength is highly complicated and is not considered pertinent to the present invention, yet is included to show the state of the art.
Rasmussen U.S. Pat. No. 4,039,364 shows merely the combination of uniaxial and biaxial plies in a criss-cross fashion to increase strength and resistance to tearing. By combining extruded laminates which have been stretched in alternating directions, a patchwork of orientation is achieved in a multi-ply laminate. This patent is highly complicated and does not lend itself to efficient, relatively inexpensive on-line operation as does the present application.
Finally, Schwarz U.S. Pat. No. 4,223,059 is provided to show the uniaxial stretching of fibers to gain increased strength in webbing. This invention is not concerned with the tearing of a film in a transverse direction to the stretching axis.
The present invention, unlike the prior art, provides a relatively simple and inexpensive process and apparatus to multi-directionally orient the films which are to be laminated. Orienting the molecular structure in a random fashion results in increased stiffness and tear resistance. Further, there being no lengthy axes of orientation, there are no axes along which tears can propagate. Consequently, the film produced by the process and apparatus described herein is greatly improved over the prior art and discloses a relatively simple and inexpensive solution to the afore-mentioned problems.